(a) Field of the Invention
The present invention relates to an orthogonal frequency division multiplexing (OFDM) communication system. More specifically, the present invention relates to an apparatus for improving a peak-to-average power ratio (PAPR) of the OFDM system, and a method thereof.
(b) Description of the Related Art
As the dissemination and use of information continues to dramatically increase, telecommunication techniques are rapidly being developed.
Requirements of various visual information multimedia services including conventional voice services have also increased, and therefore various techniques for transmitting data at high-speed have been developed in order to satisfy customers using the services. Among the techniques, the OFDM technique is being particularly applied.
The OFDM technique is for transmitting data at high-speed in a multi-path channel environment by using a plurality of subcarriers having orthogonality with each other in a given frequency bandwidth. In the OFDM technique, a signal is transmitted and received by fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) methods.
The OFDM method has high bandwidth efficiency and high effectiveness against multi-path fading, and therefore it has been widely used in the 4th generation mobile communication system (4G), terrestrial digital TV (Digital TV), and Wireless Local Area Networks (WLAN).
However, the OFDM method problematically has a high peak-to-average power ratio (PAPR) of a transmission signal because signals having a maximum value are generated in a time domain as signals having a limited range in a frequency domain are inverse-fast-Fourier-transformed.
Accordingly, the transmission signal according to the OFDM method is distorted due to the high PAPR when the transmission signal is transmitted through a nonlinear device such as a high power amplifier (HPA).
Specifically, the PAPR shows a high value when signals input to be transmitted have the same value and phase.
Accordingly, various studies for improving the PAPR in the OFDM communication system have been ongoing.
For example, a selected mapping method (SLM), a dummy sequence insertion (DSI) method, a tone reduction method (TRM), and a block coding method have been suggested to reduce the PAPR in the OFDM communication system.
In the SLM, the input signal is multiplied by various phase vectors, a signal having a lowest PAPR is detected among the multiplied signals, and the detected signal is transmitted.
That is, the signal having the lowest PAPR is selected and transmitted after one information signal is multiplied by U number of the phase vectors which are different from each other, and inverse-fast-Fourier-transformed.
Therefore, performance is proportionally increased as a number of the phase vectors is increased according to the SLM. However, IFFT operation is also increased as the number of phase vectors is increased, and a PAPR calculation process is additionally increased in the time domain.
A signal multiplied by a predetermined phase vector is transmitted by a transmission terminal, and therefore information on the phase vector is required to be added to the transmission signal so as to inform a receipt terminal of which phase vector has been multiplied by the signal in the transmission terminal.
In this case, several subcarriers are required to transmit the phase vector information. However, a data rate is reduced to be less than a usual data rate because of the subcarriers.
The more phase vectors are used, the more subcarriers are required to inform of the phase vector information. Accordingly, the data rate is further reduced.
In the TRM, a pseudo impulse is generated by using predetermined subcarriers, and a maximum peak value in the time domain is canceled out with the pseudo impulse in order to reduce the PAPR.
While fewer calculations are performed in the TRM comparing to the SLM, the data rate is reduced by the number of subcarriers used for generating the pseudo impulse.
The block coding method is for transmitting a signal by coding remaining subcarriers such that the PAPR of the signal may be reduced, and the DSI method is for transmitting a signal by inserting a dummy sequence to remaining subcarriers.
IEEE ELECTRONICS LETTERS VOL. 30 NO. 25, pp 2098-2099 (published in December, 1994) introduced a transaction about the block coding method under the title of “Block coding scheme for reduction of peak to mean envelope power ratio of multicarrier transmission schemes” by A. E. Jones et al.
Still, there is a problem in that the transmission efficiency is reduced in the block coding method and the DSI method.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention, and therefore, unless explicitly described to the contrary, it should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.